This application is based on and incorporates herein by reference Japanese Patent Applications NO. 2000-395601 filed Dec. 26, 2000, No. 2001-94176 filed Mar. 28, 2001, and No. 2001-204024 filed Jul. 4, 2001.
Field of the Invention
The present invention relates to a printed wiring board and a manufacturing method thereof, specifically relates to a double-sided printed wiring board and a multilayer printed wiring board with a plurality of electrically-interconnected conductor pattern layers formed thereon and to a manufacturing method thereof.
It is proposed that, in a printed wiring board, an insulator board includes a plurality of conductor patterns and the conductor patterns are interconnected by a conductive compound in a via-hole formed in the insulator board. As a manufacturing method for such a printed wiring board, a method shown in FIG. 12A is proposed. In this method, a substantially cylindrical via-hole 124 is formed in an insulator board 123, which is made of prepreg in B stage status prepared by impregnating a core material such as glass cloth with unset thermosetting resin. A conductive paste 150, which is an interlayer connecting material consisting of metal particles and binder resin made of unset thermosetting resin, is packed in the via-hole 124. Subsequently, the board and conductive foils 122 forming a conductor pattern are laminated.
By hot-pressing this piled body, as shown in FIG. 12B, the conductive paste 150 becomes a unified conductive compound 151 with the setting of the binder resin, and conductive foils 122 forming conductor patterns are interconnected by the substantially cylindrical conductive compound 151 formed in the substantially cylindrical via-hole 124.
In this proposed art, interconnection between the conductive foils 122 forming conductor patterns is achieved with the substantially cylindrical conductive compound 151. Therefore, in a case that the printed wiring board incurs a stress due to deformation such as bending, the conductive compound 151 is likely to incur stress concentration in the vicinity of a junction part 151b that is a junction part with the conductor pattern (the conductive foil 122). If repeated or large stress concentration is generated in the vicinity of the junction part 151b, reliability of the interconnection is lowered.
The present invention has been made in view of the above-mentioned aspects with an object to provide a printed wiring board having reliable interlayer connection and a fabrication method thereof.
To achieve the above object, a printed wiring board has a unified conductive compound in a via-hole. The compound has a side wall adjacent to an area contacting the conductor pattern. The wall has an inclination against the conductor pattern in such a manner that the farther from the conductor patterns on the side wall, the closer to the center axis of the via-hole.
Preferably, the conductive compound is formed such that the cross section thereof on the cross-sectional plane that passes the center axis of the via-hole provides an arch shape.
According to the invention, even in the case that a stress due to deformation such as bending is applied to the printed wiring board, it is possible to prevent the stress from concentrating in the vicinity of a junction part of the conductive compound. Therefore, the reliability of the interconnection avoids becoming worse. The film is made of thermoplastic resin. Therefore, when the conductive compound with the inclination of the side wall is formed, the insulator film is readily deformed plastically and the via-hole is readily formed in a shape conforming to the shape of the conductive compound.
Moreover, when the compound is formed by sintering metal particles, apparent volume is reduced. Therefore, the conductive compound is readily formed with the inclination of the side wall.
The conductor pattern is made of metal. The interlayer connecting material includes first and second metal particles. The first metal particles can form first alloy with the metal making up the conductor pattern. The second metal particles have higher melting point than the heating temperature for interconnecting layers and can form second alloy with the metal making up the first metal particle. The unified conductive compound is formed by hot-pressing the interlayer connecting material in the via-hole between a plurality of conductor patterns. Therefore, the conductor patterns are electrically interconnected with the conductive compound and the interposing solid phase diffusion layer that is formed by the mutual solid phase diffusion between the metal making up the conductor pattern and the first metal in the conductive compound.
Namely, the electrical interconnection between the conductor patterns is not made by mechanical contact so that the interlayer contact resistance hardly changes. Thus, the reliability of the interconnection is certainly enabled to avoid becoming worse.